Electric machine- modular

ABSTRACT

An electric machine ( 100 ) comprises a stator ( 112 ) and a rotor ( 114   a, b ) mounted for rotation about a rotor axis ( 120 ) with respect to the stator. Permanent magnets ( 124   a, b ) are carried by the rotor. The rotor has an output ( 190 ). The stator has coils ( 122 ) wound on stator bars ( 116 ) for interaction with the magnets. The rotor has two stages ( 114   a,b ) arranged one at either end of the stator bars, with two air gaps ( 126   a,b ) between the ends of the bars and the rotor stages. An annular housing ( 102, 142   a, b,    146 ) retains and mounts the stator. A bearing ( 164   a, b ) is between the rotor and stator, the rotor being hollow around said rotor axis. There are two significant magnetic flux paths ( 30,30 ′) of the motor. The first passes between adjacent coils in a circuit on a substantially circumferential plane with respect to the axis ( 120 ). A second path  30 ′ is in an axial plane, passing around the bearing. The stator coils are spaced around the rotor axis and approach the rotor axis no closer than a first, stator radius (R 1 ) of the stator. The bearing comprises rolling elements rolling on a surface of the rotor that is no closer to the rotor axis than a second, rotor radius (r), which rotor radius is between 60% and 90% of the stator radius.

This invention relates to a permanent magnet electric machine comprisinga stator and a rotor journalled for rotation in the stator. The statoris provided with coils wound on and the rotor is provided with permanentmagnets to cooperate with the coils across an air gap between the rotorand stator. The machine may be either a motor or a generator and is inmany embodiments an axial flux machine. In particular it relates to ayokeless and segmented armature machine (hereinafter referred to as a Ymachine).

BACKGROUND

Woolmer and McCulloch [1] describe the topology of a Y machine,discussing its advantages of reduced iron in the stator enabling animprovement in torque density. It comprises a series of coils woundaround bars spaced circumferentially around the stator, ideally axiallydisposed, (ie parallel the rotation axis of the rotor). The rotor hastwo stages comprising discs provided with permanent magnets that faceeither end of each coil of the stator. The magnetic path at any stage ofoperation is: through a first coil into a first magnet on a first stageof the rotor; across a back iron of the rotor to an adjacent secondmagnet on the first stage; through a second coil of the stator adjacentthe first coil; into a first magnet on the second stage of the rotoraligned with the second magnet on the first stage; across the back ironof the second stage to a second magnet on the second stage and alignedwith the first magnet on the first stage; and completing the circuitthrough the first coil.

One difficulty with electric machines generally is to provide adequatecooling. This is a particular problem with a Y machine having a hightorque density that significant heat is generated in the coils at hightorques and is often a limiting factor in the torques that can beemployed, at least for extended periods of time.

Another difficulty with electric machines generally is torque ripplecaused by cogging. Again, this is a particular problem with a Y machinesince the discrete coils do not overlap and indeed rely on magneticseparation, not only between adjacent coils on the stator but alsobetween adjacent magnets on the rotor. Clearly, this problem is reducedto some extent by providing different numbers of permanent magnets onthe rotor versus coils on the stator, but since magnets are aligned withone another as the “cog” between adjacent magnets engages with thecorresponding “cog” between adjacent coils there is an inevitable torqueripple.

Magnetic connection between the coils and the permanent magnets dependson a strong magnetic field being developed through the coils, either bythe magnets in the case of a generator or by the coils themselves in thecase of a motor and the permeability of the magnetic circuit should beas low as possible to permit the maximum flux density through the coils.For this purpose a high permeability core or bar is provided aroundwhich the coils are wound. However, the bar is preferably laminated orotherwise arranged to reduce the incidence of eddy currents in the bar.Also, the bars are preferably provided with shoes to spread the flux inthe air gap and reduce the flux density therein—the air gap is of highreluctance and increasing its area reduces that reluctance, which meansthat less permanent magnet material can be used. It is desirable toreduce the amount of such material to a minimum.

WO-A-2006/066740 discloses a Y machine comprising a housing having acylindrical sleeve mounting stator coils internally, the sleeve beinghollow whereby cooling medium is circulated. However, the coils areembedded in a thermally conducting material to carry heat to statorhousing. A rotor is rotatably journalled in the housing. The stator barsappear to be laminated, as they are in GB-A-2379093 that also disclosesa Y machine, as does WO-A-03/094327.

U.S. Pat. No. 6,720,688 discloses a Y machine in which the rotor acts asa vane pump to circulate fluid within a chamber defined by a statorhousing through which a rotor shaft, supported on bearings in thehousing and carrying the rotor, extends. The fluid cools stator coils.US-A-2005/0035676 discloses another Y machine, particularly adapted forgearless drive of a vehicle wheel.

US-A-2007/0046124 discloses a Y machine in which the rotor has twocircumferentially arrayed rows of alternating segments of permanentmagnets and ferromagnetic pole pieces.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the present invention, there is provided a shaftlesselectric machine comprising a stator and a rotor mounted for rotationabout a rotor axis with respect to the stator, permanent magnets beingcarried by the rotor, an output on the rotor, the stator comprisingcoils wound on stator bars for interaction with the magnets of therotor, the rotor having two stages arranged one at either end of thestator bars, with two air gaps between the ends of the bars and therotor stages, an annular housing retaining and mounting the stator; abearing between the rotor and stator, the rotor being hollow around saidrotor axis, said output comprising a flange connectable to the rotor fortransmitting. The rotor is thus not otherwise supported in or on thehousing. Being shaftless means that any component to be rotatably drivenby (or provide rotary drive to) the machine can be connected to theflange. This component may comprise a shaft, but preferably it comprisesa component that can be arranged within the confines of the rotor tosave space. Alternatively, it may comprise a universal joint housing orspider housing for providing angularly variable drive to for from ashaft. It may comprise the flange of another machine for doubling up theoutput power available.

In one exemplary embodiment, there are at least two significant magneticflux paths of the motor: a first passing through a first stator bar,across a first of the air gaps, through a first magnet on a first stageof the rotor into a back iron of the first stage, into an adjacentsecond magnet, across the first air gap into a second stator baradjacent the first, across the second air gap, through a third magnet onsecond stage of the rotor into a back iron of the second stage, into anadjacent fourth magnet, across the second air gap and back into thefirst stator bar; and a second path passing through the first statorbar, across the first air gap and through the first magnet on the firststage and into the back iron of the first stage, through the first stageinto the second stage of the rotor around said bearing into the backiron of the second stage, into the fourth magnet, across the second airgap and back into the first stator bar.

This flux path is possible if the rotor is directly mounted through saidbearing inside the stator, and on a sufficiently large diameter that thesecond flux path is sufficiently short so that the reluctance of theoverall magnetic circuit for the coils and magnets is reduced. Mountingthe rotor in this way also shortens the cantilever between the bearingthat locates the rotor and the magnets that drive the rotor throughtheir interaction with the stator coils (or are reacted by the statorcoils in the case of a generator).

However, in another embodiment:

the stator coils are spaced around the rotor axis and said rotor stagesapproach the rotor axis no closer than a first, stator radius (R₁) ofthe stator; and

the bearing comprises rolling elements rolling on a surface of the rotorthat is no closer to the rotor axis than a second, rotor radius (r),which rotor radius is between 60% and 90% of the stator radius.

With such a large diameter of the rotor bearing, and a hollow rotor,several advantages flow, starting from the ability of the rotor to berelatively stiff whereby the air gaps can be small without risk ofcontact. Alternatively, the mass of the rotor can be reduced leading toefficiency and cost savings

Preferably, the surface of the rotor on which the rolling elements rollis an integral surface of the rotor flange. Said integral surface of therotor flange is preferably hardened and may be frusto-conical, saidrolling elements then being needles. The rotor radius is in this eventthe minimum separation of the needles from said rotor axis.

The stator coils may have a radial extent (C₁) such that the smallestcircle enclosing the stator coils has a coil radius (R₂) where thestator radius (R₁) is between 50% and 80% of the coil radius (R₂). Thecoils may have a circumferential extent (C₂) that is between 50% and150% of the radial extent (C₁=R₂−R₁).

Preferably, the entire load between the output and the annular housingis transmitted through the bearing between the stator and rotor, wherebyno other mounting of the rotor or its output with respect to the statorhousing is provided. This has the advantage that the form of output canbe changed from system to system without disturbing the fundamentalarrangement of the rotor, stator and stator housing. While the term“output” used herein is appropriate for a motor, where the output isemployed to drive a load, it is inappropriate for a generator and shouldin that context be understood as an input.

Preferably, the bars are axially aligned with the rotation axis of therotor, the bearing being between two radial planes that intersect saidair gaps. Preferably, said intersects are radial.

Preferably, the bearing is between two radial planes that intersect thecoils, bars or shoes of the stator.

Preferably, the bars and coils thereon are enclosed by a stator housingthat extends between the air gaps and defines a chamber incorporatingcooling medium to cool the coils.

Said stator housing may comprise two annular plates and two cylindricalwalls, the annular plates including recesses to locate the bars withinthe chamber. Preferably, the material of the stator housing isnon-magnetic and non-conducting. However, in the case of the separateannular plates and cylindrical walls, said cylindrical walls arepreferably aluminium and said annular plates are plastics material.Preferably, said annular plates are thinned at the ends of the bar tominimize the gap between the bars and the magnets on the rotor.Preferably, said cylindrical walls are an inner and outer wall, saidouter wall having means to mount the machine and said inner wallmounting said bearing.

Preferably, the rotor stages each comprise an annular dish, whose outerrims mount said permanent magnets and whose inner rims are connectedtogether enclosing said bearings. The rotor stages are dish-shaped toincrease their rigidity in a radial plane (ie a plane perpendicular tothe rotation axis of the rotor and also, preferably, perpendicular tothe stator bars).

Preferably, the bars and coils thereon are enclosed by a stator housingthat extends between the air gaps and defines a chamber incorporatingcooling medium to cool the coils. The stator housing may include portsfor supply and drainage of said cooling medium. Preferably, the statorhousing comprises two annular plates and two cylindrical walls, theannular plates including recesses to locate the bars within the chamber.

The material of the stator housing may be non-magnetic and electricallynon-conducting. Indeed, it may be heat insulating, in which case thestator housing preferably insulates the magnets from heat generated insaid coils.

However, the stator housing is preferably thinned at the ends of the barto minimize the gap between the bars and the magnets on the rotor.

Said cylindrical walls may be aluminium and said annular plates areplastics material. They may be an inner and outer wall, said outer wallcomprising said annular housing and having means to mount the machineand said inner wall mounting said bearing.

Preferably, said stator housing includes ports for supply and drainageof said cooling medium. The cooling fluid may be pumped through themachine through an inlet near the bottom of the machine, and out of anoutlet near the top. However, the inlet and outlet may also be adjacentone another. Fluid may flow around the outer and inner radii of thecoils, some fluid also flowing between the coils. Preferably, thecooling fluid flows back and forth between the outer and inner radius onplural occasions by reason of blocks disposed between the coils and thestator housings, whereby the fluid is forced in between the coils. Theremay be between two and eight transitions of the fluid flow between thecoils. The cooling flow may alternatively be split, with some flowingaround the inner diameter of the coils from the inlet, and the restflowing at the outer diameter in the opposite direction, some fluidflowing also between the coils. Different flow paths may of course bearranged.

Preferably, the rotor stages each comprise an annular dish, whose outerrims mount said permanent magnets and whose inner rims are connectedtogether enclosing said bearing. Each said inner rim may comprise acylindrical flange with an interface for mutual inter-engagement. Aspacer may be provided between the cylindrical flanges to adjust preloadon the bearing.

The cylindrical flanges can include bosses arranged parallel said rotoraxis to receive fasteners for clamping said rotor stages together.

The output of the machine may comprise a disc and a hub. The hub mayinclude any convenient drive form such as a constant velocity hub, ormerely a splined shaft. For some ap[placations a tripod cup may beprovided. Conveniently, the disc is connectible by said fasteners tosaid bosses of the rotor. Preferably, the bearing comprises twobearings, one on either side of a flange on the stator, whereby axiallocation of the rotor stages with respect to the stator is determined.

Said annular housings may have axial interfaces enabling at least twosuch machines to be connected together, sharing a common rotor axis. Therotors of the connected machines may themselves be interconnected byfasteners passing through the bosses of adjacent rotors, a spacer beingdisposed between them. This enables greater torque capacity machines tobe provided.

The exposed ends of the machine are preferably closed by covers fittedon the annular housing, at least one having a central aperture throughwhich said output is adapted to extend.

Where the machine is a motor preferably at least two of the motors areconnected side by side, at least two of which have independent rotors,each provided with its own output. In this event, each cover is providedwith said central aperture through which the two outputs extend. Indeed,an aspect of the present invention provides a vehicle comprising a motordefined above, having a driveshaft from each output to wheels ondifferent sides of the vehicle. In this case, provided the rotors areindependent, no differential is required.

In another embodiment there is provided an electric machine comprising arotor having permanent magnets and a stator having coils wound on statorbars for interaction with the magnets across an air gap defined betweenthem, wherein the rotor has two stages arranged one at either end of thebars and wherein the bars have a shoe at each end of each bar that linksmagnetic flux through the bars with said magnets on each stage, andwherein adjacent shoes facing the same stage of the rotor have ahigh-reluctance shoe gap between them, and adjacent magnets on eachstage of the rotor have a high-reluctance magnet gap between them,wherein the shoe and magnet gaps are angled with respect to each othersuch that they engage progressively as the rotor rotates.

Preferably, the shoe on one side of each coil facing a first of said twostages is skewed with respect to the shoe on the other side of therespective coil facing the second of said two stages, and said shoe gapsbetween adjacent shoes at either ends of the bars that carry them crossthe magnet gap at different rotational positions of the rotor withrespect to the stator.

Thus, although the coil on a given bar and the magnet pair on the rotorstages are aligned, the coil at one end begins to engage the firstmagnet of the pair before the other magnet. Preferably, the skew is suchthat there is no alignment in the direction of magnetic flux of thehigh-reluctance gaps at each end of each bar.

Preferably, when viewed in an axial direction with respect to the axisof rotation of the rotor, said shoes are four-sided, with inner andouter sides being arcs or tangents of circles centred on said rotationaxis and said other sides being a leading and trailing edge of the shoe,wherein said leading and trailing edges are chords of one of saidcircles, each radius of that circle that intersects each chord and thatcircle making the same angle with the respective chord.

In yet another embodiment there is provided an axial flux electricmachine comprising a rotor having permanent magnets spacedcircumferentially on first and second stages of the rotor and a statordisposed between said stages and having coils wound on stator bars ofthe stator for magnetic interaction with the magnets across an air gapdefined between the rotor and stator, wherein the bars have a shoe ateach end of each bar that links magnetic flux through the bars with saidmagnets on each stage, and wherein the stator is a casting of at leasttwo annular components, each comprising a ring of connected shoes andone including some or all the bars or parts of them and the othercomprising any remaining bars or parts of them, said coils beingdisposed on the bars before the annular components are connectedtogether to complete construction of said stator.

Preferably, the annular components are identical. Preferably, eachcomprises half of each bar and is provided with interfaces adapted tofacilitate connection.

Preferably, said interface comprises a stud and socket, wherein the studon each bar of one component engages the socket of a facing bar on theother component.

Preferably, high reluctance gaps are provided between each shoe of eachcomponent, said gap comprising a thinning of the thickness of theannular component between said bars.

In a still further embodiment there is provided an electric machinecomprising a rotor having permanent magnets and a stator having coilswound on stator bars for interaction with the magnets across an air gapdefined between them, wherein the bars have shoes that link magneticflux through the bars with said magnets, and wherein the bars and shoesare formed separately from one another and at least a part of each isformed by moulding soft-iron particles so that the particles have ashort dimension that is arranged transverse a reluctance-plane, and thebars and shoes are assembled so that said reluctance-plane of the bar isparallel a longitudinal axis of the bar and said reluctance-plane of theshoe is transverse said longitudinal axis.

The alignment of the short dimension of the particles transverse saidreluctance-planes results in each reluctance-plane having a minimumreluctance. Preferably, said particles of at least the bars have asingle longitudinal dimension and said particles are also aligned sothat their longitudinal dimension is parallel a reluctance-direction insaid reluctance-plane, said reluctance-direction of the bars beingparallel said longitudinal axis of the bar. If the particles of theshoes have a single longitudinal dimension, preferably saidreluctance-direction is radial with respect to said longitudinal axiswhen the bars and shoes are assembled.

Said moulding of said soft-iron particles may be by pressing round softiron particles in a direction transverse to said reluctance-planewhereby the particles are flattened to produce said short dimension.Alternatively, said moulding may be of already flattened particles, orof elongate particles. Elongate particles may be aligned prior tomoulding by use of a magnetic field. Moulding includes shaping.

Preferably, the rotor has two stages arranged one at either end of thebars and shoes are provided at each end of each bar. Preferably, theelectric machine is an axial flux machine and the bars are arrangedparallel the rotor rotation axis.

The bars may include a rolled sheet of ferromagnetic material whose axisof roll is arranged parallel said longitudinal axis. The sheet itself ispreferably rolled in production in a direction parallel their roll inthe bars whereby the grains of the material are themselves oriented inthe eventual direction of flux, ie parallel said longitudinal axis. Saidroll may be disposed around a shaped soft-iron pressed-particle core,whereby the cross section of the bar perpendicular said longitudinalaxis is substantially trapezoidal. Alternatively, said roll may be thecore of a shaped annulus of pressed soft-iron particles, whereby thecross section of the bar perpendicular said longitudinal axis issubstantially trapezoidal.

In accordance with other aspects of the present invention, there isprovided electric machines that incorporate some or all of the foregoingaspects (where they are not mutually exclusive), such combinations beingevident to the skilled person. Whereas the following description ofspecific embodiments may include or exclude different aspects mentionedabove, this is not to be understood as being significant.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 is a schematic side view of a yokeless and segmented armaturemachine to which the present invention primarily (but not exclusively)relates;

FIG. 2 is a perspective view of the arrangements of FIG. 1;

FIG. 3 is a perspective exploded view of a stator housing and stator ofan electric machine;

FIG. 4 is a perspective exploded view of a stator of an electricmachine;

FIGS. 5 a, b and c are respectively an end view, a section on the lineB-B in FIG. 5 a and a perspective view of a stator of an electricmachine;

FIGS. 6 a, b c and d are respectively an exploded perspective view of astator bar and shoes of an embodiment of an electric machine, an endview of another embodiment of bar, an end view of a further embodimentof bar, both of an embodiment of an electric machine, and a perspectiveview of a composite stator bar and resultant flux paths;

FIGS. 7, 8 and 9 are respectively a cut perspective view, a slice and asection (both of the latter views being in the cut plane of FIG. 7) ofan electric machine in accordance with an aspect of the presentinvention;

FIGS. 10 and 11 a are respectively a section and a slice on the lines10-10 and 11-11 respectively in FIG. 9;

FIG. 11 b is a schematic illustration corresponding to FIG. 11 a, butwith a different coolant flow arrangement; and

FIGS. 12 and 13 are respectively a side and an end view in the directionof the Arrows XII and XIII respectively in FIG. 9.

DETAILED DESCRIPTION

A yokeless and segmented armature machine 10 is illustratedschematically in FIG. 1. The machine 10 comprises a stator 12 and tworotors 14 a,b. The stator 12 is a collection of separate stator bars 16spaced circumferentially about a rotation axis 20 of the rotors 14 a,b.Each bar 16 has its own axis 16 a which is disposed parallel to therotation axis 20. However, that is not absolutely essential. In an axialflux machine, the axis 16 a is indeed parallel the rotation axis 20.However, it can be disposed at any angle thereto, even radially withrespect to the rotation axis 20. The following discussion is in respectof an axial flux machine, but this should not be understood to belimiting in any sense and, where the context permits, the inventionequally applies to other inclinations of the stator bars 16.

Each end of each stator bar is provided with a shoe 18 a,b which servesa physical purpose of confining a coil stack 22, which stack 22 ispreferably of square section insulated wire (or possibly rectangularsection) so that a high fill factor can be achieved. The coils 22 areconnected to an electrical circuit (not shown) that (in the case of amotor) energizes the coils so that the poles of the resultant magneticfields generated by the current flowing in the coils is opposite inadjacent stator coils 22.

The two rotors 14 a,b carry permanent magnets 24 a,b that face oneanother with the stator coil 22 between. Indeed, in the axial fluxmachine, the rotors and their magnets are radially disposed, but whenthe stator bars are inclined, then they are likewise. Two air gaps 26a,b are disposed between respective shoe and magnet pairs 18 a/24 a, 18b/24 b. There are an even number of coils and magnets spaced around theaxis of rotation 20 and, preferably, there are a different number ofcoils and magnets so that each coil does not come into registration witha corresponding magnet pair all at the same time and at the samerotational position of the rotor with respect to the stator. This servesto reduce cogging.

In a motor (with which the present invention is primarily concerned) theabove-mentioned electric circuit is arranged to energize the coils 22 sothat their polarity alternates serving to cause coils at different timesto align with different magnet pairs, resulting in torque being appliedbetween the rotor and the stator. The rotors 14 a,b are generallyconnected together (for example by a shaft, not shown) and rotatetogether about the axis 20 relative to the stator 12, which is generallyfixed (for example in a housing, not shown). One advantage provided bythe arrangement is illustrated in FIG. 1 in that the magnetic circuit 30is provided by two adjacent stator bars 16 and two magnet pairs 24 a,b.Thus, no yolk is required for the stator 12, although a back iron 32 a,bis required for each rotor linking the flux between the back of eachmagnet 24 a,b facing away from the respective coils 22.

Thus, in the case of a motor, by appropriate energization of the coils22, the rotor 14 can be urged to rotate about the axis 20. Of course, inthe situation of a generator, rotation of the rotor 14 a,b inducescurrents in the stator coils 12 according to the changing magnetic fluxinduced in the stator bars 16 as the rotors 14 a,b rotate.

However, in either case heat is generated in the coils 22 and theefficiency of the machine is reduced, and its capacity limited, if thisheat is not removed. Accordingly, the present invention suggestsenclosing the stator coils 16 within a housing that extends through theair gap 26 a,b and which is supplied with a cooling medium.

Turning to FIG. 3, a stator 12 a in an embodiment is shown in which thestator coils are located between plastics material clam shells 42 a,b.These clamshells have external cylindrical walls 44, internalcylindrical walls 46, and annular radially disposed walls 48. Theannular walls 48 include internal pockets 50 to receive the shoes 18 a,bof the stator bars 16 and serve to locate the stator coil assemblies 16,22, 18 a,b when the two clam shell housings 42 a,b of the stator 12 aare assembled together. The stator housing 42 a,b defines spaces 52internally of the coils 22 and also externally at 54 around the outsideof the coils 22. Furthermore, there are spaces 56 between the coils.Although not shown in FIG. 3, when assembled, the stator housing 42 a,bis provided with ports that allow cooling medium (preferablyelectrically non-conducting liquid) to be pumped into the spaces 52, 54,56 to circulate around the coils and cool them. Indeed, being made,preferably, from a plastics material such as [polycarbonate] or otherlow heat-conducting material, heat generated by the coils and conductedinto the shoes 18 a,b is retained within the housing and not transmittedto the magnets 24 a,b, which are particularly susceptible to heat.Choice of the material employed for the clamshells 42 a,b is to someextent dependent on the design working temperature and, if this is low,many materials are suitable, but, if it is high, then heat resistantmaterial such as glass fiber reinforced plastics material would bedesirable. Further reference to the cooling arrangements of the presentinvention are also described below with reference to FIGS. 7 to 13.

A preferred arrangement involves construction of the machine asdescribed above and then, when complete, filling the spaces 52,54,56with a settable liquid resin or lacquer that wets all the internalsurfaces of those spaces, including the coils 22. Once the resin has hadthe opportunity to penetrate every space it is drained from the machineleaving only a surface coating of the resin inside the chamber definedby the spaces 52,54,56. Prior to draining, the chamber may be evacuatedin order to commit the liquid lacquer to penetrate small spaces,especially between the wires of the coils 22. When the vacuum isremoved, resumption of atmospheric pressure drives the lacquer into anyremaining unoccupied spaces. Indeed, the lacquer is preferably of lowviscosity so that it penetrates small spaces easily. After draining, theresin cures (or is cured) to form an electrically insulating layer thatseparates the spaces 52,54,56 from coils 22. By this means, water can beemployed as the cooling medium. Suitable lacquers are within theknowledge of a person skilled in the art.

Referring again to FIGS. 1 and 2, even without identical numbers ofmagnets 24 a,b and coils 22, an inherent problem of the arrangement isthe cogging effect that occurs as the high-reluctance gaps 25 betweenadjacent magnets pass over the corresponding gaps 27 between stator coilshoes 18 a,b.

It is well known that coil cores for electric machines are frequentlymade from steel laminations. Steel is an excellent conductor of amagnetic field. It provides a low reluctance path therefore and has lowhysteresis loss. However, a problem with most ferromagnetic materials isthat they are generally also electrical conductors. Therefore, thechanging flux through an electrical conductor creates eddy currents.These are minimized by employing laminations that are separated by aninsulator, with the insulation being parallel to the desired fluxdirection so that transverse electrical currents are minimized. However,a new technique is meeting with some success employing soft-ironparticles coated with insulation and moulded to a desired shape (softmagnetic composites—SMC), being bound together by the resinousinsulation. A high-pressure compaction process is used to mould thecomponent into a complex shape, capable of producing three-dimensionalmagnetic flux patterns with an excellent form factor and enabling a highfill factor winding to be employed, wound straight onto SMC teeth.

Turning to FIG. 4, a stator 12 b of an embodiment of electric machine isshown. This is a particularly suitable arrangement of the stator in alow cost arrangement. It has integral stator bars 16′ formed from two,preferably identical, components 75 a,b. Each component is an annulus 76with upstanding bar-parts 78. The bar-parts may have alternating studs80 and pockets 82 on facing interfaces 81, so that, when oriented toface one another, two identical components 75 a,b can be mated together,with the studs 80 entering the pockets 82 of the other component. Thetwo components can be glued together. However, prior to assembly,pre-wound coils 22 (shown schematically in FIG. 4 as solid rings) arelocated on the bar parts 78 of one component 75 a,b so that, whenconnected together, the components 75 a,b and the coils 22 complete anassembly of the magnetic parts of the stator 12 b.

The advantage of the arrangement shown in FIG. 4 is that the magnetsfacing the annulus 76 on each side of the stator are never presentedwith an air gap between adjacent stator coils 22. Accordingly, theinherent problem of cogging mentioned above can be eliminated, or atleast reduced—the magnets see a continuous reluctance, which can bealmost constant as a function of rotor position. However, magneticconnection between adjacent coils is to be discouraged, since that shortcircuits the flux path and reduces the efficiency of the motor.Accordingly, the annulus 76 is thinned at 84 between each bar part 78 sothat the opportunity for magnetic shorting is reduced. However, byproviding a high reluctance gap 84 between each stator coil thismitigates the anti-cogging effect of the complete metal face 76.Accordingly, there is a balance to be made between smooth running of themotor and its efficiency. Nevertheless, there is an optimum position atwhich cogging is minimized to a substantial degree without significantimpairment of motor efficiency. An advantage of the present embodimentis its potential low cost of manufacture.

The components 75 a,b are advantageously constructed from SMC material,each pressed in a single mould. However, the simplicity of their shapealso permits them to be manufactured from a single annulus of woundlaminations (having an axis of winding on the rotation axis 20), withslots 83 between adjacent bar parts 78 being cut out with a wire cutter.Finally, the advantage of the present invention could be achieved byemploying the arrangements described above with reference to FIGS. 2 and3 but where the shoes 18 and bars 16 are not constructed in a singleannulus but each independently. In this event, the shoes are sized sothat they contact one another when arranged in the motor and therebyreduce cogging.

In FIGS. 5 a and b, an alternative arrangement of the stator 12 c isshown that also reduces cogging, but without affecting the efficiency ofthe machine. Here, each stator bar 16 is provided with its own shoe 18so that there is a resultant air gap 27 a between them. Normally, thiswould result in the cogging effect mentioned above. However, here, theair gap 27 a is skewed relative to the radial direction by an angle α₁,at least one side 18 j of the shoe is skewed at this angle, the radiusin question passing through the bottom corner 18 g of the shoe. Theother side 18 h of the shoe is skewed at an angle α₂ that differ fromα₁, by a quantity dependent on the width of the air gap 27 a. Be that asit may, the average value of α₁ and α₂, is between 1° and 45°,conveniently about 10° with the number of pole pieces shown. The statorbar 16 is trapezium shaped, as in the embodiments described above, withrounded corners and the coils 22 are likewise trapezium shaped aroundthe cores formed by the bars 16. They are symmetrically disposed withrespect to the rotation axis 20. This means that at opposite corners 18d,f the coil 22 extends beyond the extremity of the shoe 18. However, atleast at the outer edge 18 e, the shoe overlaps to a small extent thecoil 22 of the adjacent shoe. The trailing corner 18 g at least overlapsthe coil 22 of its own stator bar 16.

To the right of FIG. 5 is shown, in dotted line, the air gap 27′a thatis on the opposite side of the stator 12 c, the bottom corner 18′g ofits shoe being fully visible. It can be seen, therefore, that the twoair gaps 27 a, 27′a overlap in an axial direction only in a smalldiamond shaped region 27 b. Assuming that the high reluctance gaps 25between the magnets on the rotors are radial, then the effect of skewingthe shoes is that the transition from one magnet to another from theperspective of a particular stator coil is spread over a wider arc ofrotation of the rotor with respect to the stator than if the gaps areboth radial.

Of course, it is equally feasible to skew the magnet gaps 25 and thesame effect can be achieved. That is to say, the shoe gaps 27 could beradial, as they are in the embodiments described above with reference toFIGS. 1 to 3, with the magnet gaps inclined oppositely with respect toeach rotor 14 a,b. Alternatively, a combination of skews of both thestator shoes and rotor magnets could be arranged. However, shaping themagnet is expensive, whereas the stator shoes are preferably a pressedpart that is easily shaped. In any event, it is desirable that the arcof transition, shown as the angle β (being the angle subtended betweenthe circumferential limits of the two shoe gaps 27 a, 27′a), is equal toabout the sum of α₁ and α₂. Of course, there is a balance to be struck,because the transition from one magnet to another represents a region atorque reduction and therefore spreading this has the corollary effectof concentrating the torque in between the transitions.

It is also to be noted that the shoes 18 are chamfered outwardly at 18 karound the entire periphery of the shoe. This assists in focusing theflux out of the plain of the shoes 18 towards the magnets 24 a,b.

Indeed, in an aspect of the present invention, the problem of minimizingthe reluctance of the material of the stator bar and shoe in thedirection of the magnetic flux is addressed in the arrangement of FIGS.6 a to d. Thus while SMC material is very suitable, as discussed abovewith reference to FIG. 4, it should be noted that, while coatedsoft-iron particles have the capacity to reduce eddy currents andgenerally to have a low magnetic reluctance in all directions, they donot have the best, that is to say, the minimum reluctance possible,which is still in the domain of laminations, at least in the plane ordirection of the laminations.

In this aspect, the present invention suggests employing such particlesin the construction of the stator bar 16 and shoes 18, but arrangingthem so that they have a preferential direction, or at least plane, oflow reluctance, which is prefereably lower than normally provided bysuch particles. In the case of the bar 16, this preferential directionis in planes parallel to the axis 16 a. In the case of the shoes 18, aminimum reluctance is desirably arranged in planes perpendicular to thelongitudinal axis 16 a. This can be provided in several ways, althoughfundamental is the separate construction of the bar 16 and shoes 18, asshown in FIG. 6 a, and their subsequent assembly.

Thus, the bar 16 of FIG. 6 a is manufactured from round,insulation-coated, soft-iron particles. These particles are firstflattened into disc-like components, before being placed into a mouldand finally pressed together. The mould is arranged so that thedirection of pressing of the particles, and their initial distributionprior to pressing, is such that the major dimensions of the particleslie in a plane that is parallel to the axis 16 a. This might mostconveniently be achieved, albeit only partially, by commencing withessentially round particles in the mould and pressing them together in adirection perpendicular to the axis 16 a. For example, pressing upwardlyin the direction of the Arrow A not only flattens the particles in aplane orthogonal to the direction A, but also tends to spread them inthe direction of the Arrows B.

Ideally, however, the particles are elongate and are arranged in themould with their long axis parallel to the axis 16 a. This can beachieved by employing a magnetic field to align the particles. In thatevent, the line of minimum flux for the component is not just in planesparallel to the axis 16 a, but actually in that specific direction.

On the other hand, the shoes 18 are preferably manufactured by pressinground particles in a direction parallel to the axis 16 a so that, duringthe compaction process, they spread laterally in the plane perpendicularto the axis 16 a. When the shoes 18 and bar 16 are assembled together,the magnetic flux can therefore travel with minimum reluctance throughthe bar 16 in the direction of the longitudinal axis 16 a and exit thebars 16 both in the direction of the axis 16 from the end 16 d of thebars to enter directly the air gaps 26 a,b, but also orthogonally intothe shoe peripheries 18 c, as can be seen from the magnetic flux arrowsindicated in FIG. 6 d.

In a preferred arrangement, the stator bars 16 also comprise alamination roll, which can improve the directional bias of minimumreluctance. Thus, in FIG. 6 b, a roll 90 of insulation-coated steel isarranged in a mould (not shown) with its axis parallel the (ultimate)axis 16 a of the bar 16 b to be formed. The mould is then filled withparticles that are pressed and compacted around the lamination roll sothat a plane of minimum reluctance of the particles is parallel the axis16 a. They surround the roll 90 and give the bar its desiredtrapezium-shaped section.

An alternative construction is to form a trapezium-shaped core 92 ofpressed soft-iron particles having at least a plane of minimumreluctance parallel the axis 16 a. A lamination roll 94 is then woundaround the core 92 and results in a stator bar 16 c having the desiredexternal sectional shape.

Both the bars 16 b,c of FIGS. 6 b and c each have preferentialdirections of minimum reluctance parallel to the axis 16 a. Collars 18c, formed from pressed, soft iron particles, have minimum reluctanceplanes perpendicular to the axis 16 a. When assembled, the bar andcollars result in a stator core that has an extremely low-reluctance andis directionally optimized.

The invention is further described with reference to FIGS. 7 to 13illustrating a particular construction of motor 100. Again, while amotor is described, it should be understood that the principles alsoapply directly to a generator. The motor 100 is, in fact, two motorslices 100 a,b bolted together. Each motor slice 100 a,b has a tubularhousing 102 a,b having radially planar end faces 104 a,b whereby severalhousings 102 can be bolted together end to end by bolts and nuts 106passing through bosses 108 arranged around the housings 102 a,b. Indeed,the motor 100 can be mounted in a vehicle, for instance, using thebosses 108 as mounting flanges. Despite being bolted together and beinga composite motor 100, each motor slice 100 a,b is independent of oneanother, as described further below, and can be driven at its own speedand torque, as required by a motor management system, which is notdescribed further herein. However, as also explained further below, themotor slices 102 a,b could be connected to a single output drive,thereby doubling the output torque available. Indeed, there is no limitto the number of motor slices that can be stacked together.

Thus, each motor slice 100 a,b has a stator 112 having a plurality ofstator coils 122 mounted on stator bars 116 having shoes 118 a,b. Thecoils 122 are spaced circumferentially around the rotor axis 120, asshown in FIG. 10 and there are 18 of them in the motor of FIG. 10. Eachstator coil shoe 118 a,b is received in a pocket 150 of an annularnon-electrically conducting, non-magnetic clamshell 142 a,b. Theclamshells are fixed around their outer periphery 143 a,b to internalflanges 144 a,b of the motor housings 102 a,b.

The internal edges 145 a,b of the annular clam shells 142 a,b aremounted on flanges 147 a,b of an essentially tubular inner statorhousing 146. It is to be noted that the inner stator housing component146, together with the clamshells 142 a,b and the motor housing 102complete an annular chamber 152 in which the stator coils are disposed.

Turning to FIG. 11, the motor housing 102 is provided with a port boss154, provided with an inlet 156 for cooling medium. Inside the chamber152 barriers 158 are disposed between the first coils and the housings102,146 to divide the chamber 152 into two parallel annular passages 152a,b. Each is supplied with their own respective branch 156 a,b of theinlet port 156. The parallel passages 152 a,b are separated by the coils122, between which there are gaps 155. Thus, cooling medium circulatingin the passages 152 a,b can cross and circulate around the entireperiphery of the coils 122. After completing a circuit around the motor(in a contra flow direction, it is to be noted, that will encourageturbulence through and between the gaps 155) the cooling medium exitsthe port boss 154 by outlets 160 a,b. They join at port 160 (see FIG. 9)and return the cooling medium to a pump and heat exchanger (neithershown) from whence it came. Alternative approaches are quite feasible:

1) The cooling fluid is pumped straight through the machine, with theinlet near the bottom of the machine, and the outlet near the top. Thefluid may flow around the outer and inner radii of the coils, some fluidalso flowing between the coils. This is the most simple cooling path toimplement, but probably the least effective;

2) The cooling fluid is forced to zig-zag around the motor, movingbetween the outer and inner radius on 2-8 occasions (by blocks disposedbetween the coils and the stator housings 102,146) so that the fluid isforced in between the coils, which is generally the hottest part of themachine;

3) The cooling flow is split (as described above), with some flowingaround the inner diameter of the coils, and the rest flowing at theouter diameter in the opposite direction. Some fluid flow will alsooccur between the coils; and

4) In a particularly preferred arrangement, the cooling flow is asillustrated in FIG. 11 b in which one inlet 156′ and one outlet 160′ isprovided, with blocks 158 a on either side of coil 122 a between theinlet and outlet. Blocks 158 b are periodically disposed around themachine firstly (158 b 1) and lastly (158 b 2) on the outside of coils122 b,c and between at least one block 158 c on the inside of coil 122d. By this arrangement the flow enters the inlet 156 and begins aroundthe outside of the machine, but is directed by the first block 158 b 1to transition to the inside of the chamber 152, between different onesof intervening coils 122 d. From there, flow continues circulationaround the machine but is forced by block 158 c to transition back tothe outside of the chamber. Further around the machine, block 158 b 2obliges transition back to the inside and, finally, in order to exit themachine through outlet 160, blocks 158 a force transition a final timeback to the outside. In FIG. 11 b, there are four transitions. However,any even number of transitions is possible, or even an odd number if theinlet and outlet are arranged one on the outside of the machine (asshown) and the other on the inside (not shown).

Turning to FIGS. 8 and 9, the inner stator housing 146 has a centralinternal flange 162 on either side of which are disposed bearings 164a,b. The bearings 164 a,b mount rotors 114 a,b. The rotors are connectedtogether across internal flanges 166 a,b. These are tubular and areprovided with spaced bosses 168 to receive nuts and bolts 170 thatconnect the two rotors 114 a,b together. Thus, the rotors 114 a,b are,to all intents and purposes, a single, integral structure. Extendingfrom the cylindrical flanges 166 a,b are dish-shaped wings 172 a,b thatterminate in an annular section 174 a,b on which magnets 124 a,b aremounted. Indeed, the extensions 174 a,b are preferably provided withpockets 176 to receive the magnets and firmly locate them.

Between the magnets 124 a,b and the clamshells 142 a,b are air gaps 126a,b. As will be well understood in motor technology, the air gaps shouldbe as small as possible in order to reduce the reluctance of themagnetic circuit. However, the arrangement of the motor described withreference to FIGS. 7 to 13 permits a very narrow air gap to beengineered by virtue of the few manufacturing tolerances that have to beaccommodated in assembly of the motor 100 a,b. Because the bearings 164a,b represent a significant source of lost motion the rotors are adaptedto apply a pre-stress to the bearings, which pre-stress is limited by aspacer 180 disposed between them. Of course, the axial dimension of thespacer can be honed to ensure a tight fit. However, apart from thebearing there are relatively few other components whose tolerances stackup and necessitate a large air gap, Of course, one such component is thestator 112 itself, for which the dimensions of the flanges 147 a,b ofthe inner stator housing 146, and the depending flanges 144 a,b, as wellas the dimensions of the clamshells 142 a,b, are critical in ensuringthe smallest possible air gap 126 a,b, despite the presence of a wallformed by the clamshell being included therein. Moreover, it is apparentthat any stresses in the rotor will result in torsional (that is, aboutaxes perpendicular to the rotation axis 120, or in linear stresses inthat direction) that must be accommodated by the stator 112. However,the series of stator bars and shoes spanning the chamber 152 providesignificant diagonal reinforcement within the chamber 152 to render theinner housing 146 extremely secure in an axial direction.

Furthermore, the concept of mounting the rotor 114 directly in thestator 112 has two further beneficial effects. The first is connectedwith the general principle of the motor design which demands that themagnets 124 and coils 122 be disposed as far as possible from therotation axis 120 so that the magnetostrictive force acting between thecoils and magnets translates into maximum torque about the rotationaxis. This means, however, that, if the fixing of the rotor with respectto the stator is at distance that is not much less than the radius ofthe magnets/coils, the rotor must be very rigid over that distance. Bymounting the rotor directly on the stator that distance is reduced andtherefore the rotor need not be so rigid. Alternatively, the air gap canbe smaller. Secondly, by connecting the rotor using a dish shapedannulus 172 that transforms into a tubular body 166, a further returnpath 30′ (see FIG. 8) for the magnetic flux is created. At least, thisis the case if the rotor is made from a ferromagnetic material. Thisadditional flux path is advantageous because it reduces the requirementfor the flux to confine itself to a circumferential direction in theflanges 174 between magnets but also permits an alternative return pathfor each magnet-coil-magnet circuit. The overall reluctance of themagnetic circuit is thereby reduced.

It should be appreciated that the axial force applied to each rotor dueto the magnets is significant, and it increases as the air gap reducesand may be of the order of 7500 N per rotor. As a result of this, theaxial support of the rotors is extremely important and thus thebearing(s) between the stator and rotor need to provide a strong andstable reaction to this force. If the rotors are perfectly located oneither side of the stator, there is a net axial force of zero, but toachieve this requires tight build tolerances and a stiff bearingassembly. However, by mounting the rotor directly inside the stator asdescribed herein, that accuracy is achievable within reasonable cost.The flange 162 against which the bearings seat and locate, axially, iscritical in this regard.

Indeed, with reference to FIGS. 8 and 11, there are certain geometricalfeatures of an embodiment of a machine according to one aspect of thepresent invention. As mentioned above the coils 112 have an externalradius R₂. By that is meant the radius of the smallest circle thatencompasses all the coils. Likewise, they have an internal radius R₁,which correspondingly is the radius of the largest circle that fitswithin the confines of all the coils. The coils are sensibly arranged ina circle around the rotor axis 120, but that is not absolutely required.However, the radius r of the bearings 164 a,b, being here the radius ofthe circle just touching the innermost part of the rolling elements ofthe bearings, is arranged as large as possible and is preferably relatedto stator radius R₁ by the expression:

r=k ₁ *R ₁

where k₁ is between 0.5 and 0.9 in value.

Indeed, the coils have radial (C₁) and circumferential (C₂) extents,where

C ₁ =R ₂ −R ₁.

Although the circumferential extent can be anything, it is defined asthe centre-to-centre arc, centred on the rotor axis 120, betweenadjacent coils. However, one convenient motor has the followingrelationships:

R ₁ =k ₂ *R ₂; and

C ₁ =k ₃ *C ₂

Where k₂ is between 0.5 and 0.8, and k₃ is between 0.75 and 2.0.

In fact, the relationship may be taken further such that:

r=k*R ₂, where

k=*k ₁ k ₂.

k preferably has a value between 0.3 and 0.6, and may be about 0.45 inone suitable arrangement.

Although the bearings 164 a,b are shown as ball bearings having theirown races, the design permits bearing surfaces to be formed onrespective frusto-conical or cylindrical surfaces of the inner statorhousing 146 and cylindrical flanges 166, and for taper roller bearings,confined to a cage, to be disposed between them. This can result in eventighter tolerances being achieved. As mentioned above, the rotorcomponents are constructed from a ferromagnetic material such as steeland may be cast or forged and machined as required. However, the innerstator housing 146, and indeed the motor housing 102, is convenientlycast from non-magnetic material such as aluminium (alloy). Evenaluminium can have a hardened bearing surface however. In this event, aflange 162 is not employed. In any event, the present design enables anair gap in the order of 1.0 mm (±0.1 mm) to be maintained at minimummanufacturing cost.

As mentioned above, the two motors 100 a,b are independent. The rotors114 are not connected to each other. However, they clearly could be, bydisposing an appropriate spacer between them, and extending the bolts170 so that they pass through both rotors. Indeed, there is nothing toprevent further motors being added in series, so that three or moremotors could be employed in tandem. As can be seen in the drawings, thesides of the composite motor are closed by covers 178 that are a pressfit inside internally-cylindrical extensions 102 c,d of the motorhousings 102. The covers are dished pressings and are a press fit insidethe extensions 102 c,d although other methods of fixing are conceivable.They have a central opening through which a motor out put 190 extends.

The output 190 comprises any suitable component and may be a shaft.Here, it is shown as a standard drive hub having a tripod-cup 192 forreception of a shaft (not shown) having a three-lobed yoke. A seal (notshown) would normally be disposed between the cover 178 and the hub 190to isolate the internal environment of the motor 100. The hub 190 isconnected by an annular disc 194 to the rotor 114. The disc 194 issecured to the rotor by the bolts and nuts 170, and to the hub 190 bybolts (not shown) in apertures 196 in the hub 190. Indeed, it is anaspect of the direct mounting of the rotor on the stator that the outputconfigurations possible without any disturbance of the motor design ispossible. Thus, the shaft-less topology allows for a wide variety ofoutput configurations, including:

-   -   Automotive “constant velocity” (CV) joint housing;    -   Splined shaft (either male or female); and    -   Flat drive plate with any hole pattern.

In one application, in which the motor 100 illustrated in FIGS. 7 to 13is particularly intended, the motor is arranged to drive two vehiclewheels. A further motor could be arranged to drive other pairs of wheelsin multi-axle vehicles. The motor would be arranged substantiallycentrally between the wheels with drive shafts extending from each ofthe two drive hubs 190 a,b. There would be no requirement for anydifferential, because each motor-slice could be driven independentlywith constant torque. The machine in this arrangement may operate asboth a motor and a generator, particularly in hybrid vehicles, butcertainly at least when employing regenerative braking.

As is evident from the description above, the covers 178 are merely dustexcluders and protect the internal components of the machine 100. Theyhave little if any structural role. The structural connections between afixture (such as a vehicle in which the machine is disposed) and theoutput are as follows. The fixture is connected to the motor housing.The motor housing structurally mounts the stator. The statorstructurally, although also rotationally, mounts the rotor. The rotorstructurally mounts the output, which is not otherwise structurallysupported by the motor housing. Here, the term “structurally” is beingused in the sense that the mountings are the main or only mountings forthe component in question. In many known scenarios, for example, ahousing mounts a stator and also (rotationally) mounts a rotor. It couldbe suggested therefore that the stator mounts the rotor. However, suchmounting is incidental and is not what is meant herein by structuralmounting through the substantially exclusive agency of the component inquestion. Of course, in that regard, a seal disposed between the cover178 and the hub 190 neither “mounts” the hub on the cover, let alonestructurally, and does not disturb the fundamental structural mountingof the hub in the housing through the agency of the rotor and stator.

It can be seen that by mounting the rotor directly on the stator at adistance from the rotation axis, a substantial hollow space is createdinside the rotor. Depending on the application, this provides anopportunity to dispose a gearbox, particularly a planetary gearbox,inside the motor. To some extent, in many circumstances with a machineof the present design, a gear box is not necessary because theelectronics required to manage the coils can enable the machine tooperate at a substantially constant maximum torque (subjectsubstantially only to cooling limitations) over a wide range of speeds,for example torques of 500 Nm per motor slice, to rotational speeds inexcess of 3000 rpm, are feasible. Nevertheless, this option isdistinctly available.

This arrangement also has the advantage of facilitating interconnectionof machines in tandem, because there is no requirement to disturb thejournal arrangement of the rotor in the housing as would normally be thecase where the rotor is supported in bearings fixed in the housing.Clearly, there is some scope for debate as to where a stator begins anda housing in which it is fixed ends. Indeed, the invention provides, inmotor terms the following non-exclusive list of options:

(a) A single 500 Nm slice with a spline output;

(b) Two independently-controlled 500 Nm slices, each with their ownCV-type output for automotive applications;

(c) Four slices joined as two pairs (1000 Nm per pair), each pair with aCV-type output, again potentially for (high performance) automotiveapplications;

(d) Four slices fixed rigidly together giving 2000 Nm;

Reference is made to FIG. 8, in which the bottom half differs from thetop half of the drawing by virtue of the rotors 114 a,b beinginterconnected by bolts 170 a that extend through aligned bosses,although spaced by spacer sleeve 169. In fact, there is no reason whythere should not be two outputs, as previously described, provided nodifferential is required, but in the lower half of FIG. 8, left-handcover 178′ is completely closed, and left-hand rotor 114 a does not havea disc 194 and hub 190 connected to it. Instead drive is to (or from) asingle hub1 90 and disc 194 connected to the right-hand rotor 114 b.Indeed, the four slice motor (not shown) can be made simply bycontinuing the addition of rotors 114 leftwardly in FIG. 8.Alternatively, a dual-drive four-slice motor can be achieved simply bymirroring the arrangement in the bottom half of FIG. 8, removing thecontinuous covers 178′ and connecting the annular housings 102 of eachpair together.

Another element to note is that a slice doesn't necessarily have to be afurther motor slice 100 a,b—it could be a separate gearbox slice toprovide an alternative torque-speed balance. So, in example (a) above,the slice could be added to an epicyclic gearbox that steps therotational speed down by, for example, a factor of 4:1. This wouldreduce the maximum output speed but will conversely give 2000 Nm torque(500 Nm×4) from a very light weight assembly. Of course, these figuresapply to the topology illustrated in FIGS. 7 to 13 employing 18 statorpoles and 20 magnets per rotor. However, other options are of courseavailable, in either direction, down to 300 Nm and upwards of 1000 Nm.

Although the motor 100 of FIGS. 7 to 13 is shown without the features ofthe embodiments described above with reference to FIGS. 4 to 6, thosefeatures can advantageously be incorporated, as desired. Of course, theembodiments described with reference to FIGS. 4 and 5 are mutuallyexclusive.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of them mean “including but notlimited to”, and they are not intended to (and do not) exclude othermoieties, additives, components, integers or steps. Throughout thedescription and claims of this specification, the singular encompassesthe plural unless the context otherwise requires. In particular, wherethe indefinite article is used, the specification is to be understood ascontemplating plurality as well as singularity, unless the contextrequires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith. All of the features disclosed in this specification(including any accompanying claims, abstract and drawings), and/or allof the steps of any method or process so disclosed, may be combined inany combination, except combinations where at least some of suchfeatures and/or steps are mutually exclusive. The invention is notrestricted to the details of any foregoing embodiments. The inventionextends to any novel one, or any novel combination, of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), or to any novel one, or any novel combination,of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

REFERENCES

-   [1] T J Woolmer and M D McCulloch “Analysis of the Yokeless and    Segmented Armature Machine”, International Electric Machines and    Drives Conference (IEMDC), 3-5 May 2007

1. A shaftless electric machine comprising a stator and a rotor mountedfor rotation about a rotor axis with respect to the stator, permanentmagnets being carried by the rotor, an output on the rotor, the statorcomprising coils wound on stator bars for interaction with the magnetsof the rotor, the rotor having two stages arranged one at either end ofthe stator bars, with two air gaps between the ends of the bars and therotor stages, an annular housing retaining and mounting the stator; abearing between the rotor and stator, the rotor being hollow around saidrotor axis, said output comprising flange connection means by which aflange is connectable to the rotor for transmitting rotary power to orfrom the rotor.
 2. An electric machine as claimed in claim 1, in whichthere are at least two significant magnetic flux paths of the motor: afirst passing through a first stator bar, across a first of the airgaps, through a first magnet on a first stage of the rotor into a backiron of the first stage, into an adjacent second magnet, across thefirst air gap into a second stator bar adjacent the first, across thesecond air gap, through a third magnet on second stage of the rotor intoa back iron of the second stage, into an adjacent fourth magnet, acrossthe second air gap and back into the first stator bar; and a second pathpassing through the first stator bar, across the first air gap andthrough the first magnet on the first stage and into the back iron ofthe first stage, through the first stage into the second stage of therotor around said bearing into the back iron of the second stage, intothe fourth magnet, across the second air gap and back into the firststator bar.
 3. An electric machine as claimed in claim 1, in which thestator coils are spaced around the rotor axis and said stator coilsapproach the rotor axis no closer than a first, stator radius (R₁) ofthe stator; and the bearing comprises rolling elements rolling on asurface of the rotor that is no closer to the rotor axis than a second,rotor radius (r), which rotor radius is between 60% and 90% of thestator radius.
 4. An electric machine as claimed in claim 3, in whichthe surface of the rotor on which the rolling elements roll is anintegral surface of the rotor flange.
 5. An electric machine as claimedin claim 4, in which said integral surface of the rotor flange ishardened.
 6. An electric machine as claimed in claim 4, in which thesurface is frusto-conical, said rolling elements are needles
 7. Anelectric machine as claimed in claim 2, in which the surface of therotor on which the rolling elements roll is an integral surface of therotor flange and the surface is frusto-conical, said rolling elementsare needles, and said rotor radius is the minimum separation of theneedles from said rotor axis.
 8. An electric machine as claimed in claim3, in which the stator coils have a radial extent (C₁) such that thesmallest circle enclosing the stator coils has a coil radius (R₂) wherethe stator radius (R₁) is between 50% and 80% of the coil radius (R₂).9. An electric machine as claimed in claim 8, in which the coils have acircumferential extent (C₂) that is between 50% and 150% of the radialextent (C₁=R₂−R₁).
 10. An electric machine as claimed in claim 1, inwhich the entire load between the output and the annular housing istransmitted through the bearing between the stator and rotor, whereby noother mounting of the rotor or its output with respect to the statorhousing is provided.
 11. An electric machine as claimed in claim 1, inwhich the bars are axially aligned with the rotation axis of the rotor,the bearing being between two radial planes that intersect said airgaps.
 12. An electric machine as claimed in claim 11, in which said airgaps are radial.
 13. An electric machine as claimed in claim 1, in whichthe bearing is between two radial planes that intersect the coils, barsor shoes of the stator.
 14. An electric machine as claimed in claim 1,in which the bearing is between two radial planes that intersect thecoils of the stator.
 15. An electric machine as claimed in claim 1, inwhich the bars and coils thereon are enclosed by a stator housing thatextends between the air gaps and defines a chamber incorporating coolingmedium to cool the coils.
 16. An electric machine as claimed in claim15, in which said stator housing comprises two annular plates and twocylindrical walls, the annular plates including recesses to locate thebars within the chamber.
 17. An electric machine as claimed in claim 15,in which the material of the stator housing is non-magnetic andelectrically non-conducting.
 18. An electric machine as claimed in claim15, in which the material of the stator housing in the air gaps is heatinsulating.
 19. An electric machine as claimed in claim 15, in whichsaid stator housing is thinned at the ends of the bar to minimize thegap between the bars and the magnets on the rotor.
 20. An electricmachine as claimed in claim 16, in which said cylindrical walls arealuminium and said annular plates are plastics material.
 21. An electricmachine as claimed in claim 16, in which said cylindrical walls are aninner and outer wall, said outer wall comprising said annular housingand having means to mount the machine and said inner wall mounting saidbearing.
 22. An electric machine as claimed in claim 1, in which therotor stages each comprise an annular dish, whose outer rims mount saidpermanent magnets and whose inner rims are connected together enclosingsaid bearing.
 23. An electric machine as claimed in claim 22, in whicheach said inner rim comprises a cylindrical flange with an interface formutual inter-engagement.
 24. An electric machine as claimed in claim 23,further comprising a spacer between the cylindrical flanges to adjustpreload on the bearing.
 25. An electric machine as claimed in claim 23,in which said cylindrical flanges include bosses arranged parallel saidrotor axis to receive fasteners for clamping said rotor stages together.26. An electric machine as claimed in claim 1, in which said flangeconnection means comprises an annular radial face of the rotor includingfasteners for connection of said flange.
 27. An electric machine asclaimed in claim 26, in which the flange comprises a disc and a hub. 28.An electric machine as claimed in claim 26, in which said disc isconnectible by said fasteners to said bosses of the rotor.
 29. Anelectric machine as claimed in claim 1, in which the bearing comprisestwo bearings, one on either side of a flange on the stator, wherebyaxial location of the rotor stages with respect to the stator isdetermined.
 30. An electric machine as claimed in claim 1, in which saidannular housings have axial interfaces enabling at least two suchmachines to be connected together sharing a common rotor axis.
 31. Anelectric machine as claimed in claim 25, in which the bearing comprisestwo bearings, one on either side of a flange on the stator, wherebyaxial location of the rotor stages with respect to the stator isdetermined and the rotors of the connected machines are interconnectedby fasteners passing through the bosses of adjacent rotors, a spacerbeing disposed between them.
 32. An electric machine as claimed in claim1, in which the exposed ends of the machine are closed by covers fittedon the annular housing, at least one having a central aperture throughwhich said output is adapted to extend.
 33. An electric machine asclaimed in claim 1 being a motor.
 34. A motor as claimed in claim 33, inwhich the bearing comprises two bearings, one on either side of a flangeon the stator, whereby axial location of the rotor stages with respectto the stator is determined and at least two of the motors are connectedside by side, at least two of which have independent rotors, eachprovided with its own output.
 35. A motor as claimed in claim 31, inwhich the bearing comprises two bearings, one on either side of a flangeon the stator, whereby axial location of the rotor stages with respectto the stator is determined and the rotors of the connected machines areinterconnected by fasteners passing through the bosses of adjacentrotors, a spacer being disposed between them and each cover is providedwith said central aperture through which the two outputs extend.
 36. Avehicle, comprising a motor as claimed in claim 34 and a driveshaft fromeach output to wheels on different sides of the vehicle.